EP0633435A1

EP0633435A1 - Condenser for air-conditioning systems, in particular for motor vehicles

Info

Publication number: EP0633435A1
Application number: EP93830293A
Authority: EP
Inventors: Maurizio Parrino
Original assignee: Borletti Climatizzazione SRL; Magneti Marelli Climatizzazione SpA
Current assignee: Denso Thermal Systems SpA
Priority date: 1993-07-06
Filing date: 1993-07-06
Publication date: 1995-01-11
Anticipated expiration: 2013-07-06
Also published as: DE69302682T2; DE69302682D1; EP0633435B1; ES2087702T3

Abstract

A condenser for air-conditioning systems, particularly for motor vehicles, comprising a group of tubes (14) secured to a pack of substantially flat fins (16) by means of mechanical expansion of the tubes (14) following their insertion in holes aligned in the fins (16).
The tubes (14) have an oblong cross-section without any flat walls, with a ratio of between 2.5 and 3.75 between the maximum dimension (b) and the minimum dimension (a) of the cross-section, and a ratio of between 15 and 30 between the maximum dimension (b) of the cross-section and the thickness (s) of the tube (14); and in that the ratio between the distance between the fins (p) and the thickness of each fin (t) is less than or equal to 11.

Description

The present invention relates to an air conditioning system, in particular for motor vehicles.
More precisely, the invention relates to a condenser of the type defined in the preamble of Claim 1.
In the motor vehicle field, the working conditions of a condenser may be rather harsh, especially in terms of operating temperature and pressure. Frequently, the internal pressure of the tubes, on the Freon side, reaches values of more than 25 bar and temperatures of more than 120°C; when pressures of this type reach values of more than 27 - 28 bar, proposed protective systems provide for the uncoupling of the compressor from the mechanical drive and thus switch off the air conditioning system.
Supply specifications generally require that the condenser should not be subjected to permanent deformations in any part with pressures inside the tubes and the distributors of the order of 30 bar (test pressure), whilst the condenser should not burst at pressures equal to twice the test pressure (approximately 60 bar).
Such harsh working conditions have forced condenser manufacturers to develop essentially two different techniques for the construction of the heat exchanger portion of the condenser. These two construction techniques concern the methods of constructing the so-called core of the condenser, that is, the heat exchanging part formed of tubes and fins. These techniques are:-

mechanical enlargement or expansion; and
braze welding.

In expanded or mechanically assembled condensers, the heat exchanger network consists of round tubes, made of copper or aluminium, and of flat fins generally made of aluminium and, more rarely, of copper. The tubes are inserted in the holes in the fins and are successively deformed plastically by means of a suitable expansion tool which increases the diameter of the tubes, such that the initial play between the tubes and the holes in the fin is taken up. At the end of the expansion step, a radial interference between the tubes and fins of the order of 0.1 - 0.2 mm is obtained. The final expansion of the tube is considered to be correct when, in addition to the plastic deformation of the tube, elastic deformation of the fins is obtained, such that stable and efficient contact between the two parts is ensured. The characteristic components of this construction process are:

structural simplicity;
simplicity of the process;
reliability;
low investment costs; and
low cost of the final product.

Braze-welded condensers are generally formed of flat tubes with internal fins made of extruded aluminium. The internal fins of the tube have a structural function, that is, they prevent the collapse of the tube when subjected to high pressures. The tube is folded in a serpentine configuration and corrugated or ondulated fins are inserted between each pair of adjacent limbs of the tube. During the braze welding process, a special aluminium alloy, which melts at a temperature which is slightly lower than the melting temperature of the aluminium parts to be connected, welds the parts placed in contact without effectively melting them. In addition, the process employs a material which enables the film of aluminium oxide, which forms and would otherwise hinder the final welding of the parts, to be destroyed. In order for the process to be efficient, the temperatures used and the furnace timing, together with the cleanness of the surfaces of the parts to be welded must be controlled. This process is therefore characterised by:

a high level of cleanness of the parts;
careful control of the process temperatures;
the use of materials coated with brazing alloy;
a degree of reliability which is not always high;
high investment costs; and
a high cost for the final product.

The two techniques for producing condensers have very different features and costs; the technique is selected on the basis of the bulk, the performance and the costs of the final product. The main restriction of current mechanically expanded condensers is their performance: their heat exchange performance is limited and they have a high pressure drop on the air side. When these aspects become predominant, it is then necessary to use braze-welded condensers which, even though they may be as much as 50% more expensive, enable the required level of performance to be attained with less bulk.
Within the field of radiators for cooling internal combustion engines and radiators for heating motor vehicle passenger compartments, the use of oval and flat tubes assembled with flat fins in accordance with the mechanical expansion or enlarging technique has already been proposed. The use of oval tubes according to the mechanical expansion assembly technique has enabled a technique of this type to be reintroduced with remarkable results in terms of overall performance and costs. However, it should be stressed that this has only occurred with the change from the oval - round shape with a ratio which is less than 2.5 between the largest dimensions and the smallest dimensions of the cross-section of the tube to an oval - flat shape with a ratio which is equal to or greater than 2.5 between the largest dimensions and the smallest dimensions. In fact, radiators and radiator cores with oval tubes and with a ratio of less than 2.5 between the maximum dimensions and the minimum dimensions of the tube cross-section, and of which the performance results were certainly not excellent and were very different from those of braze-welded radiators, were produced even as early as the second half of the Eighties.
It was only with the introduction in the Nineties of oval - flat tubes with a ratio of more than 2.5 between the maximum and minimum dimensions of the tube cross-section that the change in generations enabling expanded radiators to compete with braze-welded radiators in all respects was attained. In fact, the oval - flat tube permits a greater development of the surface area for the exchange of heat on the tube side, together with the drop in pressure on the air side, bringing it very close to the geometry of braze-welded radiators, in which the tubes are effectively flat.
Nevertheless, it should be pointed out that, although condensers belong to the general category of heat exchangers, owing to their working conditions, they cannot be likened directly to radiators. In fact, within the field of radiators for cooling engines and for heating cars, the operating pressures on the liquid side do not generally exceed 3 bar, for which reason the use of oval tubes does not involve any problems in terms of structural resistance. This has enabled tubes with a high ratio (greater than 3.75) between the maximum and minimum dimensions of the tube cross-section to be used whilst distances between the walls of the tube which are equal to or less than 0.4 mm are maintained.
However, the widely differing and known construction techniques in the field of radiators cannot automatically be applied to the field of condensers, because of the extreme working conditions to which the latter are subjected, in particular because of the high pressures which can be reached on the refrigerant fluid side.
The object of the present invention is to provide a condenser produced according to the technique in which tubes are mechanically expanded, having improved performances and dimensions ultimately comparable to those of a braze-welded condenser.
In accordance with the present invention, this object is achieved by a condenser having the characteristics which are the subject of claim 1
Further characteristics and advantages of the present invention will become clear from the following detailed description, given purely by way of non-limiting example and with reference to the appended drawings, in which:

Figure 1 is a perspective view of a condenser according to the present invention;
Figure 2 is a schematic view showing the distribution of the flow of refrigerant fluid inside the condenser in Figure 1;
Figure 2a is a view on an enlarged scale of the part indicated by the arrow II in Figure 2; and
Figure 3 is a section along the line III-III in Figure 1

heat exchanger network

tubes

flat metal fins

tubes

fins

The ends of the tubes 14 projecting from the exterior of the pack of fins 16 are welded to distributors designated 18, 20, 22 and 24.
Figure 2 shows the distribution of the flow of refrigerant fluid inside the tubes 14. The fluid in the vapour state enters the distributor 18, passes through a first group of tubes and reaches the second distributor 20 to which the tubes of a second group lead through which a flow of refrigerant fluid passes in the opposite direction to the first. The flow of fluid reaches the third distributor 22 and from there it passes, by means of a third group of tubes, to an outlet distributor 24 from which refrigerant fluid in the liquid state is drawn. The number of tubes in the above-mentioned three groups decreases progressively so as to take account of the reduction in volume of the refrigerant fluid as it passes from the vapour state to the liquid state.
The condenser according to the present invention is provided with tubas with cross-sections having an elongate oval shape. There are two possibilities for the securing of the oval tubes to the distributors 18 - 24:

a) the oval ends of the tubes 14 are welded directly in suitable slots in the distributors 18 - 24; in this case, the free part of the tube between the last fin and the distributor is preferably less than 10 mm long;
b) the ends of the tubes projecting from the pack are shaped with a circular or substantially circular profile by means of known processes and are welded then successively to the distributors 18 - 24.

In both cases, a plate, which is approximately 1 mm thick, which covers the final fin, and which is inserted before the fin pack / tube-expansion process, can be used to restrict the force on the outermost fins. This plate can also be used to protect the external fins when the tubes are welded directly to the distributors using a flame.
As stated above, in the case of condensers for air-conditioning systems for motor vehicles, pressures of the order of 25 - 27 bar are reached, even though they are transient, with temperatures exceeding 120°C; the condenser must withstand levels of this nature without undergoing structural deformation of any kind.
In general, a heat-exchanger network having oblong tubes and flat fins cannot withstand the above operating conditions, unless specific dimensional parameters having a critical role are complied with.
Following numerous laboratory tests, the Applicants have determined a series of parameters and a limited range of values within which these parameters must be kept in order to be able to produce an expanded condenser with tubes having an oblong cross-section.
In the following, the parameters a, b, s, p and t, having the following meanings:

a: = minor axis of the tube
b: = major axis of the tube
s: = tube thickness
p: = distance between the fins, and
t: = fin thickness,

The dimensional parameters critical for the dimensioning of the condenser according to the invention are the ratios b/a, b/s and p/t.
The parameter b/a expresses the ratio between the axes of the oval tube and provides information with respect to the geometry of the tube as it moves away from the circular shape. The parameter b/s relates the major axis of the tube to its thickness, expressing the rigidity of the tube. Finally, the parameter p/t expresses the ratio between the distance between the fins and their thickness. This value stresses the fundamental contribution the fins make to the structure of the condenser for withstanding the considerable pressures coming into play.
On the basis of tests performed by the Applicants, it is demonstrated that the parameters which ensure the structural resistance of the tube / fin assembly are the following:

$2.5 < = b/a < = 3.75$

$15 < = b/s < = 30$

$p/t<= 11.$
If all these parameters are respected simultaneously, the tube / fin structure can withstand the extreme pressure and temperature conditions on the fluid side without undergoing permanent deformation.
On the basis of the tests performed, a novel condenser is designed, of the type which is mechanically expanded and has oval tubes which have a flattened profile and are arranged in a single row, and has the following dimensions:

pack thickness: 18 mm
flat oval tube: b = 12, a = 3.2 and s = 0.6
fin thickness: t = 0.12 mm
distance between tubes: 12 mm
fin density: 70 - 80 fins/dm.

The critical dimensional ratios for this condenser are as follows:

$b/a = 3.75$

$b/s = 20$

$p/t < = 10.5$

The choice of producing the single-row condenser is due to the need to reduce as far as possible the drop in pressure on the air side and thus to the need to reduce the thickness of the condenser on the air side.
By way of example, the Applicants have performed pressure- tightness tests at 30 bar on heat exchanger networks of various shapes, with the following results.
A heat exchanger network having flat tubes of dimensions 12 x 3.5 x 0.35 mm, with fins which are 0.08 mm thick, and having a fin density of 90 fins/dm ( $b/a = 3.42$
; $b/s = 34.28$
; $p/t = 13.88$
) gives a notable deformation of the tubes, showing that the structure with tubes having parallel walls and flat fins cannot withstand the pressure values imposed.
A heat exchanger network having oval - flat tubes of dimensions 14.9 x 4.5 x 0.35 mm, with fins which are 0.1 mm thick, and having a fin density of 90 fins/dm ( $b/a = 3.31$
; $b/s = 42.57$
; $p/t = 11 .1$
) gives large-scale permanent deformation of the tubes with a notable change in the shape of the fins.
A heat exchanger network with oval - flat tubes of dimensions 12.2 x 3.4 x 0.4 mm, with fins which are 0.1 mm thick, and having a fin density of 90 fins/dm ( $b/a = 3.59$
; $b/s = 30.5$
; $p/t = 10.5$
): a slight permanent deformation of the tubes of approximately 0.2 mm along the minor axis is noted.
Furthermore, it is confirmed that in all cases in which the critical parameters b,/a, b/s and p/t fall within the abovementioned intervals, permanent deformations, which can be measured with conventional measuring instruments, are not obtained.

Claims

A condenser for air-conditioning systems, particularly for motor vehicles,comprising a group of tubes (14) secured to a pack of substantially flat fins (16) by means of mechanical expansion of the tubes (14) following their insertion in holes aligned in the fins (16), characterised in that the tubes (14) have an oblong cross-section without any flat walls, with a ratio of between 2.5 and 3.75 between the maximum dimension (b) and the minimum dimension (a) of the cross-section, and a ratio of between 15 and 30 between the maximum dimension (b) of the cross-section and the thickness (s) of the tube (14); and in that the ratio between the distance between the fins (p) and the thickness of each fin (t) is less than or equal to 11.